Electro-assisted Anaerobic Digestion of Waste Activated Sludge for Biomethane Production Enhanced by Hyperosmosis of Sodium Chloride: Characteristics and Microbial Mechanisms

This first-attempted study explored the hyperosmosis of sodium chloride (NaCl) for enhancing methane production from waste activated sludge (WAS) via electro-assisted anaerobic digestion. The 0.6-0.9g/g TS levels of sodium chloride caused an obvious WAS solubilization with ca. 104.7-110.3mg/L of soluble chemical oxygen demand (SCODMn) release within 96-h (including 132.56-158.34mg/L of soluble protein and 55.67-66.36mg/L of soluble carbohydrates). The NaCl addition increased conductivity and current density positively, but decreased microbial biodiversity, community evenness and archaeal metabolic activities negatively in the electro-assisted anaerobic digestion process. The proper use of NaCl for enhancing methanogenesis of WAS should seek a balance between such positive and negative impacts, which was at ca. 0.6g/g TS adding level and maintained a good-cooperation between acidogens and methanogens. Acidogens of Acrobacter, Mesotoga, Mesoaciditoga, Candidatus-cloacamonas, Bacteroidales, Bacteroidetes, Anaerolineaceae, Treponema and Fusibacter contributed to VFAs generation, while Methanobacterium, Methanomassiliicoccus and Methanothrix were responsible for methanogenesis positively under NaCl-hyperosmosis circumstances. Finally, a mechanistic illustration for depicting biomethane conversion in electro-assisted anaerobic digestion with sodium chloride addition was proposed. The results obtained might shed lights on this efficient and low-cost strategy for WAS treatment and resource recovery.